Metallic aluminum is produced industrially by electrolysis of alumina in solution in an electrolyte bath primarily made up of cryolite, using the Hall-Héroult process. The electrolyte bath is contained in an electrolysis cell, comprising a steel container coated on the inside with refractory and/or insulating materials, and at the bottom of which a cathodic unit located.
Anodes, typically made of carbonaceous material, are partially immersed in the electrolyte bath. Each anode is provided with a metal rod designed to connect it electrically and mechanically with an anode framework that is mobile in relation to a gantry fixed above the electrolysis cell.
As the anodes are consumed by electrolysis, they must regularly be changed. For this purpose, a spent anode with its rod is detached from the anode frame and moved towards a temporary support where it is allowed to cool before being moved on to an installation in which the unconsumed carbonaceous material is recovered.
However, a spent anode from an electrolysis cell emits fluorinated gases which can be harmful for man and the environment. The emissions increase in proportion to the temperature of the anode and gradually decrease at the same time as the temperature.
For many years it has therefore been attempted to prevent fluorinated gases emitted by the spent anodes from being diffused into the environment, or at least to limit this diffusion. Various devices and processes have been imagined.
In particular, one suggested process involves placing the spent anode on its support inside a closed box designed to prevent the uncontrolled outlet of fluorinated gases. According to a first known embodiment, in particular described in document FR 2 754 832 (corresponding to US document U.S. Pat. No. 5,961,812, filed in the name of Aluminium Pechiney), the gases within the box are sucked out to a treatment unit. According to another known embodiment, in particular described in document DE 42 21 882, filed in the name of Westfalia Becorit Industrietechnik, the box includes a filter containing alumina, which is able to trap fluorinated gases.
According to a third known embodiment, presented in document WO 2008/048844 filed in the name of Alcoa, the box comprises an opening through which the anode rod passes and which is equipped with a flexible sealing element working in conjunction with said rod
Another known process is described in document WO 2003/042618 filed in the name of Norsk Hydro according to which it is planned to use a storage box for the spent anode in conjunction with an extraction system.
The above mentioned processes and devices are not entirely satisfactory. It is indeed difficult, if not impossible, to obtain good sealing by means of a box, especially as the latter undergoes deformations during its lifespan, which lead to an increase in gas leaks. The means of sealing generally put forward to compensate for these problems are consumables which wear quickly and which therefore lead to additional cost. In addition, in certain configurations of the box, an in-draft moving towards the box can generate a chimney effect causing the fluorinated gases to be drawn out through the top of the box, without passing through a filtration or suction system.
Another known process described in U.S. Pat. No. 6,161,307 filed in the name of Alcan International Limited involves plunging and moving the spent anode in a fluidized alumina bath to cool the spent anode quickly and to limit the time of fluorinated gas emission. Such a process is difficult to implement and has the disadvantage of encouraging the combustion of the spent anode by a constant supply of air onto the surface of the spent anode.
In a context of overall reduction of fluorinated emissions in aluminum plants, and especially when changing anodes, the present invention aims at curing the drawbacks mentioned above, by providing a more reliable and more effective process than the processes of prior art.